This invention relates to methods for superplastic forming and diffusion bonding continuous structures and more particularly to methods of superplastic forming and diffusion bonding of Y shaped support structures called Y stringers in a simple manner which avoids lines or zones of weakness.
The process of superplastic forming and diffusion bonding is well known in the art. The process has the advantages of forming complex shapes and permitting deep drawing of metallic parts. At the temperatures involved the deformation stresses are relatively low, which permits the forming of complex parts under pressures which keep tool deformation and wear at a minimum.
Diffusion bonding which is used in conjunction with superplastic forming refers to the metallurgical bonding or joining of metal by the steps of applying heat and pressure for periods of time sufficient to cause co-mingling of the metal atoms at the places where the surfaces are joined.
Generally speaking, the process begins with cleaning the metal sheets to be used in a face-to-face or sandwich type construction. The sheets are then selectively and judiciously coated with a material commonly referred to as "stop-off" which typically is boron-nitride, yttria or some rare earth or other very inert material. The purpose of the "stop-off" is to prevent diffusion bonding of the sandwich-like material where the stop-off material has been applied. The areas coated with "stop-off" can be superplastically formed into a desired shape by the application of gas pressure applied to the inside of the metallic sandwich.
The superplastic forming and diffusion bonding process provides many manufacturing and structural advantages over more conventional manufacturing processes and is particularly useful in the formation of a variety of intricately-shaped structures including the deep drawing of parts. In the aero-space industry the process is used for forming frames, bulkheads and air passage panels. In the formation of such structures, stiffening elements are provided for strength purposes. The larger of such stiffening elements are sometimes called "frames". Typically, there is another stiffening element cross-wise, e.g., perpendicular to the frame sometimes referred to as "stringers".
This invention relates to a method of forming a relatively flat plate and a Y shaped element depending therefrom called a Y stringer.
In the past the Y stringers have been formed by joining two separate pieces by fastener elements. The Y in turn is joined to the plate member by additional fasteners.
Another proposed solution is to extrude the Y shaped support member and attach it in a conventional manner such as by rivets to the plate member. This proposal requires forming holes for the rivets and the rivets themselves. Conventional extrusion techniques also limit the shape of the parts that can be made.